The worldwide distribution of HTLV infection, the mechanism of its transmission and its role in various types of T cell malignancies and patients with acquired immune deficiency syndrme (AIDS) and pre-AIDS has been extensively studied. A highly sensitive ELISA technique has been developed and extensively used for detection of HTLV infection. Seroepidemiological studies on the distribution of HTLV-III show tht 95% of the AIDs and pre-AIDS patients, and approximately 45% of healthy homosexuals carry HTLV-III antibodies. High incidence (65%) of HTLV-III antibodies has also been found in sera of Ugandan children collected in 1972. Detection of HTLV-III antibodies in sexual partners of AIDS and ARC cases in New York suggest heterosexual transmission of HTLV-III. Other seroepidemiological studies show that populations at risk for development of AIDS include Canadian and Japanese hemophiliacs, Haitian immigrants to New Guinea, Zairians, Rwandese and male prostitutes in Singapore. The sera from AIDS and pre-AIDS patients has been shown to precipiate HTLV-III envelope proteins of 41,000, 120,000 and 160,000 daltons. HTLV-III inoculations into chimpanzees show that these animals seroconvert and virus has been reisolated from their peripheral blood indicating infection of these animals. None of these animals have so far developed AIDS-like syndrome.